I agree about the warm mist/vaporizer vs. cool mist/ultrasonic. The EPA has a run down on the differences:
http://www.epa.gov/iaq/pubs/humidif.html ... opinions differ but all things considered I much prefer the warm.
Vaporizers are incredibly simple devices, so the best one is really the one that's easiest to clean, IMO, and possibly the one with a big tank if you're running it on high all the time or something. I look at each one in the store and try to find one with a fill-hole in the tank that's big enough to fit my hand in to scrub it out.
If mold is growing, you may not need a humidifier...do you know the relative humidity in the room where you're using it? I'd get a meter (probably Home Depot or Lowes would have one) to check it...you want it between 40% and 60%. 60%'s a little high, but in the winter I'll push it up that high, generally speaking 50%'s good to shoot for.
That and regular cleaning should keep you mold free.
